As is well known, optical waveguide communication systems typically require network monitoring apparatus that provides the systems operator with information on the status of the system. See, for instance, G. Livescu, OSA 1998, Technical Digest Series, Vol. 4, Integrated Research, Victoria, B.C., Canada, pp. 43-45.
It is also well known that most optical waveguide communication systems are functionally bidirectional, but with signal flow in a given waveguide being in one direction only, i.e., a given waveguide link is unidirectional. See, for instance, the above cited Livescu reference. Herein we will refer to such systems as "bidirectional systems with unidirectional links" or "BS/ULs".
Monitoring apparatus for use in BS/ULs is known. Such systems may include one or more of optical spectrum analyzers, multi-wavelength meters, tunable Fabry-Perot cavity devices, tunable acousto-optic devices, waveguide arrays, or bulk grating spectrometers. Recently, a new device that is useful for monitoring applications in BS/ULs was disclosed. It will be referred to as a "unidirectional dispersive waveguide tap" or "UDWT". See co-assigned U.S. patent applications Ser. No. 08/741,439, filed Oct. 31, 1996 now U.S. Pat. No. 5,832,156, and an application with attorney docket number T. A. Strasser 18-12, filed Jun. 1, 1998 now U.S. Pat. No. 6,002,822, a continuation-in-part application of the referenced '439 application. See also U.S. patent application Ser. No. 08/978,352 now U.S. Pat. No. 5,850,302, filed Nov. 25, 1997, a division of the above-referenced '439 application, and U.S. patent application with attorney docket number T. A. Strasser 19-13 Ser. No. 09/088,567, filed Jun. 1, 1998, a continuation-in-part application of the above-referenced '352 application. See also J. L. Wagener et al., ECOC 97, September 1997, Conference Publication No. 448, pp. 65-68; and C. K. Madsen et al., 1998 Technical Digest Series Vol. 4, Integrated Photonics Research, Mar. 20-Apr. 1, 1998, Victoria, B.C., Canada. All of the above cited applications and references are incorporated herein by reference.
Most optical waveguide communication systems are BS/UL systems. However, truly bidirectional waveguide communication systems (i.e., systems wherein at least a portion of a given waveguide link carries signals in both directions) are also of interest. See, for instance, K. I. Suzuki et al., Electronics Letters, Vol. 33(23), November 1997, pp. 1967-1968; F. Khaleghi et al., IEEE Photonics Technology Letters, Vol. 8(9), September 1996, pp. 1252-1254; U.S. Pat. Nos. 5,742,416; 5,689,594; and 5,602,665. These references disclose bidirectional systems, but do not disclose or suggest any bidirectional monitoring devices. For instance, FIG. 2 of the Suzuki et al. paper shows a WDM system wherein signals propagate in both directions over the fiber link between the two circulators, but does not show any bidirectional monitoring device. Truly bidirectional waveguide communication systems will herein be referred to as "bidirectional" systems.
Monitoring apparatus is needed in bidirectional waveguide communication systems for substantially the same reasons as in BS/UL systems. However, in a bidirectional system monitoring apparatus advantageously is bidirectional, i.e., responsive to the signal stream in one direction as well as to the signal stream in the opposite direction, since availability of bidirectional monitoring apparatus can translate into simpler (and thus cheaper and/or more reliable) bidirectional systems. However, few if any bidirectional monitoring devices are known to date.
In response to the need for bidirectional monitoring devices, this application discloses such a device, and bidirectional systems that comprise such a device.
Glossary and Definitions
By "guided modes" we mean herein the propagating modes in the waveguide. The guided mode in a single mode conventional optical fiber is the LP.sub.01 mode.
By "cladding modes", or "bound cladding modes" we mean herein optical modes of the waveguide structure that have an effective refractive index less than the refractive index of the cladding material of the waveguide. These modes are bound, in the sense that the optical power in these modes is always localized around the waveguide, and is not spreading out in a direction orthogonal to the propagation direction.
By "radiation modes" we mean herein optical modes that are not completely localized to the waveguide structure. Radiation modes spread away from the waveguide structure, such that at some point along the length of the waveguide there is an arbitrarily small amount of optical power located in the waveguide structure.
By a "non-guided mode" we mean herein a mode other than a guided mode, e.g., a cladding mode or a radiation mode.
A refractive index grating herein is "chirped" if the (optical) repeat distance .LAMBDA. of the index perturbations is not constant as a function of the axial coordinate z of the fiber, i.e., if .LAMBDA.=.LAMBDA.(z).
A refractive index grating herein is "blazed" if the plane of the index perturbations in the waveguide is not perpendicular to the propagation direction of the guided mode or modes.